Skip to main content

Full text of "Handbook Of Chemical Engineering - I"

See other formats


PYROMETRY TABLE 8.—COLD JUNCTION COKRECTION FACTORS
431
Engelhard "LeChatelier"		Johnson Matthey "LeChatelier"		Copper-constantan		Iron-cons tan tan	
Temperature, degrees Centigrade	K	Temperature, degrees Centigrade	K	Temperature, degrees Centigrade	K	Temperature, degrees Centigrade	K
265-   450	0.65	250-   400	0.60	0- 50	1.00	0-    100	1.00
450-   650	0.60	400-    550	0.55	50- 80	0.95	100-   600	0.95
650-1,000	0.55	550-   900	0.50	80-110	0.90	600-1,000	0.85
1,000-1,450	0.50	900-1,450.	0.45	110-150	0.85	Chromel-alumel	
				150-200	0.80	0-    800   1.00	
				200-270	0.75	800-1,100    1.05	
				270-350	0.70		
temperature of the cold junction by any of the above methods is frequently troublesome since this temperature may vary considerably in practice over a few hours. There are several methods for obviating the necessity of applying such corrections and of making manual adjustments. The head of the couple may be fitted with a water jacket through which water flows at practically constant temperature from the water main. Copper wires lead from the terminals of the couple inside the water jacket to the indicator, and the pointer of the indicator on open circuit is set once for all to read the mean temperature of the water, usually 10 to 20°C. If the potentiometer indicator is used the zero adjustment slide or the cold junction dial is set at this temperature..
Compensating Leads.—The use of compensating lead wires from the couple to the indicator is in general the most satisfactory method for minimizing the cold-junction errors in industrial installations. For base-metal couples these lead wires are of nearly the same materials as those employed in the couple. For example, with a chromel-alumel couple one lead wire is chromel which is connected to the chromel wire of the couple. The other lead wire of alumel is connected to the alumel wire of the couple. Small stranded wires are used for flexibility. Thus the cold junction is carried away from the head of the couple, where the temperature varies, to a point at some distance from the furnace where the temperature is reasonably constant, and from this point copper wires lead to the indicator. The compensating wires may terminate in a thermostated cold-junction box or may be buried underground. At a depth of 10 ft. beneath the floor of a large building the temperature remains constant to within 2°C. throughout the year. Usually this mean temperature is about 12°C.^ for temperate climates but may differ from this value somewhat if the location is in the immediate vicinity of a large furnace. To use this method for controlling the temperature of the cold junction, an iron pipe of the proper length, closed at the bottom is driven into the ground and the two cold junctions, for example copper-alumel and copper-chrome!, well soldered and carefully insulated are threaded to the bottom of the pipe in such a manner as to be conveniently remov-